Process for the preparation of a paraffin isomerization catalyst

ABSTRACT

Process for preparation of a paraffin isomerization catalyst comprising a mixture of a Group IVB metal oxide, a Group VIB metal oxide, a Group IIIA metal oxide and a Group VIII metal, said process comprising the steps of (a) contacting a hydroxide of the Group IVB metal with an aqueous solution of an oxyanion of the Group VIB metal to provide a mixture, (b) drying the mixture to provide a dry powder, (c) kneading the dry powder with a Group IIIA hydroxide gel to form a paste, (d) shaping the paste to form a shaped material, (e) calcining the shaped material, (f) impregnating the calcined material with an aqueous solution of a Group VIII metal salt to provide the catalyst, (g) calcining the catalyst.

The present invention relates to the preparation of a paraffin isomerization catalyst comprising mixed aluminium and zirconium oxides modified with tungsten oxide and a hydrogenation/dehydrogenation component of a Group VIII metal. The catalyst is useful for the production of high-octane gasoline from a hydrocarbon feed stream comprising C₄₊ hydrocarbon cuts.

BACKGROUND OF THE INVENTION

Multi-branched paraffins are ideal gasoline-blending components possessing high octane numbers. For environmental reasons, there is a need to find substitutes for aromatic components in gasoline. Therefore, there is an incentive to develop a process for increasing the octane number of the C₄-C₁₂ cuts. While C₅/C₆ paraffin isomerization is a common refinery process, commercialisation of processes including higher fractions (C₇₊ hydrocarbons) meets significant difficulties given by the usually high degree of cracking to gaseous products, which is undesirable.

An article by K. Arata and M. Hino in Proceedings 9^(th) International Congress on Catalysis (1988, vol. 4, p. 1727-1735) describes a catalyst based on a Group IVB metal oxide such as zirconia in particular, modified by an oxyanion of the Group VIB, particularly tungstate and its use in paraffin isomerization.

Further research activities have shown that the catalytic performance of tungstated zirconia catalysts in paraffin isomerization may be improved by addition of a hydrogenation/dehydrogenation function, preferentially a noble metal, to the mixed solid acid catalysts. The use of tungstated zirconia promoted with noble metal in paraffin isomerization has been described in the open literature, for instance by S. L. Soled, S. Miseo, J. E. Baumgartner, W. E. Gates, D. C. Barton and E. Iglesia, Proc. 13^(th) Int. Conf. Catal. (The Taniguchi Foundation, Kobe, Japan, 1994) p. 17; E. Iglesia, D. G. Barton, S. L. Soled, S. Miseo, J. E. Baumgartner, W. E. Gates, G. A. Fuentes and G. D. Meitzner, Stud. Surf. Sci. Catal. 101 (1996) 533; G. Larsen, E. Lotero, S. Raghavan, R. D. Parra and C. A. Querini, Appl. Catal. A 139 (1996) 201.

A series of U.S. patents on solid acid isomerization catalysts have been assigned to Mobil Oil Corporation. U.S. Pat. No. 5,510,309 provides a method for preparing an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal. An example of this acidic solid is zirconia modified with tungstate. This modified solid oxide may be used as a catalyst, for example, to isomerise C₄ to C₈ paraffins. The modified solid oxide is prepared by co-precipitating the Group IVB metal oxide along with the oxyanion of the Group VIB metal. After filtration, the co-precipitate is calcined at 825° C.

U.S. Pat. No. 5,780,382 provides a method for preparing an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal.

U.S. Pat. No. 5,854,170 describes the preparation of a noble metal containing an acidic solid catalyst by impregnation of a Group IVB metal hydroxide or hydrated oxide with an aqueous solution comprising an oxyanion of a Group VIB metal. The noble metal (preferentially Pt) may be added by co-impregnation with the oxyanion of the Group VIB metal or in a separate impregnation step.

U.S. Pat. No. 6,080,904 describes a C₄-C₈ isomerization process utilising an isomerization catalyst with a hydrogenation/dehydrogenation component (preferentially Pt) and with solid acid component comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal.

In all the above-mentioned patents, aluminium is mentioned merely as a conventional matrix material such as alumina, silica-alumina and silica, with preference given to silica.

The tungstated zirconia system has frequently been described as catalyst for C₅₊ isomerization. The catalysts typically contain tungsten oxide in a concentration below 20 wt % and hydrogenation component is platinum. However, the catalytic selectivity towards isomerization delivered by these materials is not sufficient. The following patents are variations of the above-mentioned prior art.

U.S. Pat. No. 5,422,327 describes a catalyst composition of a Group VIII metal incorporated into a support consisting of zirconia, said support being further impregnated with a mixture of silica and tungsten oxide and its use in paraffin isomerization.

U.S. Pat. No. 5,648,599 claims a catalytic isomerization process comprising contacting a C₅₊ feed under isomerization conditions with a catalyst composition consisting of a Group VIII metal and a zirconia support impregnated with tungsten oxide and silica.

U.S. Pat. No. 5,837,641 describes an isomerization reaction over tungstated zirconia and the promotional effect of water on this catalyst.

U.S. Pat. No. 6,767,859 describes an alkane isomerization process using a catalyst composition of a metallic oxide doped by a metal dopant, a Group IVB metal and a hydrogenation/dehydrogenation function, the metal-doped metallic oxide being prepared by co-precipitation from solution and the metal dopant being incorporated into the crystal lattice of the metallic oxide by calcination at sufficiently high temperatures. The metal dopant incorporation into the crystal lattice of the metallic oxide is verified by X-ray diffraction. This catalyst composition shows high activities towards alkane conversion, but suffers from the disadvantage of a low selectivity towards alkane isomerization and a high cracking selectivity towards gaseous C₁-C₄ products with low commercial value.

The general objective of the invention is to provide a process for the preparation of a catalyst which is suitable for improving the octane number of a C₄₊ hydrocarbon mixture through isomerization, without substantial cracking of the produced multi-branched hydrocarbons to gaseous products.

BRIEF SUMMARY OF THE INVENTION

The above objective is achieved by process whereby a paraffin isomerization catalyst is prepared, the catalyst comprising a combination of three oxides chosen from a Group IVB metal oxide, a Group VIB metal oxide and a Group IIIA metal oxide. The catalyst is useful for the production of high-octane gasoline from a hydrocarbon feed stream comprising C₄₊ hydrocarbon cuts.

The invention comprises a process characterised by the contents of claim 1.

DETAILED DESCRIPTION OF THE INVENTION

The catalyst prepared by the process of the invention is a solid acid catalyst suitable for improving the octane number of a C₄₊ hydrocarbon mixture through isomerization, without substantial cracking of the produced multi-branched hydrocarbons to gaseous products.

Multi-branched isomers are in this case defined as compounds containing more than one carbon atom bonding to at least three other neighbouring carbon atoms or containing at least one carbon atom bonding to four neighbouring carbon atoms. Mono-branched isomers are defined as compounds containing just one carbon atom with bonds to three neighbouring carbon atoms.

The catalyst prepared according to the process of the invention can be applied to C₇₊ fractions or cuts containing this fraction (e.g. C₄-C₇, C₅-C₉, C₇-C₉, C₆-C₇, C₇, C₈ or C₉, C₇-C₁₂) and consisting mainly of paraffins and possibly naphthenes, aromates and olefins.

The process of the invention comprises the steps of

(a) contacting a hydroxide of a Group IVB metal with an aqueous solution of an oxyanion of a Group VIB metal to form a mixture,

(b) drying the aforementioned mixture. Optionally the dried mixture comprising oxides of Group IVB metal and Group VIB metal can be calcined at 600-800° C.

(c) kneading the dry powder with a Group IIIA hydroxide gel to form a paste,

(d) shaping the paste to form a shaped material,

(e) calcining the shaped material,

(f) impregnating the calcined material with an aqueous solution of a Group VIII metal salt to form the catalyst,

(g) calcining the catalyst.

Drying can be carried out by for instance spray drying. Shaping can be carried out by for instance extrusion. The catalyst is calcined at a temperature sufficiently high to decompose the Group VIII metal salt.

The process of the invention has several advantages:

-   -   preparation of the catalyst is carried out using inexpensive raw         materials, thus providing economic advantages over catalyst         preparations from halogenated starting materials.     -   use of metal halide containing starting materials is avoided,         thereby avoiding extensive procedures for the removal of halide         anions from the catalyst material. The exclusion of halide         anions on the catalyst material provides technical advantages,         since this avoids corrosion problems in the technical equipment         of the isomerization reactor units and increases catalyst         lifetime.     -   alumina is present in the catalyst in sufficient amounts to         provide mechanically strong catalyst extrudates. A catalyst         composition comprising a Group VIII metal on mixed oxides         behaves differently from noble metal supported on tungstated         zirconia or on alumina only.

The catalyst prepared according to the process of the invention comprises a carrier based on a combination of three oxides chosen from a Group IVB metal oxide, a Group VIB metal oxide and a Group IIIA metal oxide. The carrier is then impregnated with a Group VIII metal to provide the final catalyst.

In a preferred embodiment of the invention, the spray dried mixture comprising oxides of Group IVB metal and Group VIB metal is calcined at 600-800° C.

In a preferred embodiment of the invention, the catalyst prepared by the inventive process comprises zirconia, alumina and tungsten oxide, which are calcined and impregnated with a Group VIII metal.

In an embodiment of the invention the process is suitable for preparing a catalyst comprising mixed aluminium and zirconium oxides modified with tungsten oxide and a hydrogenation/dehydrogenation component of a Group VIII metal.

The presence of a Group IIIA metal oxide such as alumina in the catalyst improves the mechanical strength of the catalyst. The preparation of mechanically stable sulphated zirconia catalysts by addition of alumina is described in patent literature, for example in U.S. Pat. No. 6,326,328. In the catalyst prepared by the process of the invention, an alumina content of at least 10 wt % is required to achieve sufficient strength. The higher the amount of alumina in the catalyst, the higher is the mechanical strength of the prepared extrudate. In a preferred embodiment, the catalyst contains between 20% and 50 wt % of alumina.

The most typical aluminium source is hydrated aluminas, like pseudoboehmite. Aluminium halides such as AlCl₃.6H2O are preferably avoided because of their corrosive nature towards large scale steel equipment and their higher cost as compared to pseudoboehmite.

The quality of the Group IVB metal oxide is very important for the total catalyst performance. In an embodiment of the invention a group IVB metal oxide such as zirconium tetrahydroxide can be applied. Zirconium hydroxide can be prepared, for example, by precipitation of zirconyl nitrate with ammonia at high pH followed by heating under reflux, similarly to A. Calafat, Stud. Surf. Sci. Catal. 118 (1998) 837. An example of zirconium hydroxide suitable for use in the process of the invention has a particle size of 10 μm and possessed a surface area above 300 m²/g.

In an embodiment of the invention the Group VIB metal oxide is an oxide of tungsten. The most typical tungsten precursor is ammonium metatungstate, which is useful due to its high solubility and low price.

The Group VIII metal may be selected from any of the Group VIII metals and mixtures thereof. The preferred metals are palladium and platinum with a concentration between 0.01 wt % to 5 wt %, most preferentially between 0.05 wt % to 1 wt %.

In an embodiment of the invention in which the catalyst prepared according to the inventive process comprises mixed aluminium and zirconium oxides modified with tungsten oxide and a hydrogenation/dehydrogenation component of a Group VIII metal, the process comprises the following steps:

(a) Spray drying a mixture of zirconium tetrahydroxide suspended in an aqueous solution of ammonium metatungstate. Optionally the spray dried mixture comprising oxides of zirconium and tungsten can be calcined at 600-800° C.,

(b) Kneading a mixture of spray dried product (zirconium and tungsten oxides/hydroxides) with alumina gel and pseudoboehmite to a paste,

(c) Shaping the paste by extrusion to form an extrudate,

(d) Calcining the extrudate at 600-800° C., most preferentially at 625-700° C. to form the catalyst,

(e) Impregnation of the calcined material with the Group VIII metal,

(f) Calcination of the resulting catalyst at 300-500° C., preferentially between 350° C. to 450° C.

In a preferable embodiment of the invention the spray dried mixture comprising oxides of zirconium and tungsten is calcined at 600-800° C.

An important parameter for the activity and selectivity of the catalyst composition of this invention is the calcination temperature for the mixed zirconium/tungsten/aluminium oxide carrier. It has been found that calcination of the carrier at 750-800° C. leads to a highly active catalyst. However, the selectivity of this catalyst towards cracking to undesired gaseous C1-C4 components is high. Calcination of the zirconium/tungsten/aluminium oxide carrier between 600 and 700° C. results in a catalyst materials with lower activity, but also with a significantly higher selectivity towards liquid isomerization products and lower selectivity towards undesired cracking products. A catalyst with a high selectivity towards isomerization provides therefore significant economic advantages compared to state-of-the-art catalysts.

The overall lower catalyst activity can be compensated for by increasing the temperature for the isomerization reaction. This increases the catalytic conversion without significantly increasing the cracking selectivity.

Typical operating conditions are temperatures between 150° C. to 300° C., total pressures varying between 1 and 100 bar and liquid space velocities (LHSV) between 0.1 to 30 h⁻¹. The preferred conditions are temperatures between 130-250° C., LHSV=0.5-5 h⁻¹, pressures between 5-50 bar and a hydrogen:hydrocarbon ratio between 0.1 and 5.

The feed may optionally also include shorter paraffins, aromatics or cycloparaffins. When passing such a feed through the reactor bed, shorter paraffins are also isomerised, while aromatics are hydrogenated to the corresponding cycloalkanes. The reaction rate for ring opening will typically be very slow.

Specific embodiments of the invention for the production of a high liquid yield of gasoline with a high research octane number (RON) are described in more detail below.

The cracking factor is defined as the ratio between the sum of the weight of gaseous products (C₁-C₄) and the sum of the weight of the isomers 2,2-dimethylpentane (2,2-DMP), 2,4-dimethylpentane (2,4-DMP) and 2,2,3-trimethylbutane (2,2,3-TMB)

EXAMPLES Example 1

Spray Drying of Zirconium Hydroxide and Ammonium Metatungstate was Carried Out as Follows:

12.0 kg (NH₄)₆H₆W₁₂O₄₀ were dissolved in 180 litre demineralised water. 28.0 kg Zr(OH)₄ were mixed with the solution. The mixture was dried in a spray drier with an inlet temperature of 250° C., an outlet temperature of 90° C. and a feed flow of 29 kg/h.

Example 2

Preparation of an Isomerization Catalyst with 22 Wt % of Alumina:

1.432 kg of the spray dried product obtained in Example 1 was mixed with 1.400 kg alumina gel (pseudoboehmite 30%) and 112 g pseudoboehmite powder for 10 min and extruded as 1/16″ cylinders. The extrudates were dried at 110° C. over night and calcined at 650° C. for 3 h. 0.5 wt % Pd was introduced by incipient wetness impregnation of an aqueous [Pd(NH₃)₄](NO₃)₂ solution. The thus obtained catalyst was calcined at 400° C. for 6 h in a flow of air (4 l/min·kg catalyst) before it was placed into the reactor.

Example 3

Catalytic Isomerization:

Prior to the catalytic isomerization experiment, the catalyst was reduced with H₂ (200 Nml/min) at 200° C. and atmospheric pressure. Heptane isomerization with the catalyst prepared in Example 2 was performed in a fixed-bed reactor at a total pressure of 6 atm and with a LSHV of 2.03 h⁻¹. The feed consisted of a hydrogen/heptane mixture with a molar ratio of 4.95:1. Catalytic results are shown in Table 1.

Example 4

Preparation of an Isomerization Catalyst with 14 Wt % of Alumina:

1.00 kg of spray dried product obtained in example 1 was mixed with 1.00 kg alumina gel (pseudoboehmite 30%) for 10 min and extruded as 1/16″ cylinders. The extrudates were dried at 110° C. overnight and calcined at temperatures between 600° C. and 800° C. for 3 h. 0.5 wt % Pd was introduced by incipient wetness impregnation of an aqueous [Pd(NH₃)₄](NO₃)₂ solution. The thus obtained catalyst was calcined at 400° C. for 6 h in a flow of air (4 l/min·kg catalyst) before it was placed into the reactor.

Example 5

Catalytic Isomerization Test:

Prior to the catalytic experiment, the catalyst was reduced with H₂ (200 Nml/min) at 200° C. and atmospheric pressure. Heptane isomerization with the catalyst prepared according to Example 2 was performed in a fixed-bed reactor at a total pressure of 6 atm and with a LSHV of 2.1 h⁻¹. The feed consisted of a hydrogen/heptane mixture with a molar ratio of 4.9:1. Catalytic results are shown in Table 1.

The example shows the dependence of the catalytic C₇ conversion and the cracking factor on the carrier calcination temperature. The lower the temperature, the lower is the catalytic C₇ conversion and the lower is the cracking factor.

Example 6 Comparative Example

To 52.7 g zirconyl chloride (ZrOCl₂.8H₂O, 30% solution in hydrochloric acid) was added concentrated NH₄OH (aq) until the solution pH was approximately 9. The resulting slurry, Zr(OH)₄, was filtered and washed with 500 g of distilled, deionized water. The solid was air-dried at 130° C. for 16 hours. The dried product (11.9 g) was impregnated via incipient wetness with 8.9 g of an aqueous solution containing 2.12 g of ammonium metatungstate, (NH₄)₆H₆W₁₂O₄₀. The resulting material was dried in air and then calcined at 825° C. in air for 3 h. 7.0 g of the dried product were impregnated by incipient wetness by an aqueous solution of 55 mg (NH₄)₂PdCl₄. The catalyst was then dried at 300° C. in air for 2 hours.

Example 7

Catalytic Isomerization Test:

Prior to the catalytic experiment, the catalyst was reduced with H₂ (200 Nml/min) at 200° C. and atmospheric pressure. Heptane isomerization with the catalyst prepared in Example 6 was performed in a fixed-bed reactor at a total pressure of 6 atm and with a LSHV of 1.14 h−1. The feed consisted of a hydrogen/hydrocarbon mixture with a molar ratio of 4.41:1. Catalytic results are shown in Table 1.

Example 8 Comparative Example

0.161 mol of ZrOCl₂.8H₂O was mixed with 0.0082 mol of AlCl₃.6H₂O in a 600 ml glass beaker. 320 ml of H₂O were added to dissolve the salts with stirring. Then 25% NH₄OH was added dropwise to the solution under vigorous stirring until the final pH of the precipitation mixture reached 9.0. After stirring for more than 1 h, the precipitate was washed by distilled water and recovered through centrifugation. The material was washed 6 times to remove the chloride ions. The precipitate was dried in an oven at 120° C. over night. Then, an aqueous solution of ammonium metatungstate (3.13 g) was added to the mixed hydroxide via the incipient wetness technique. After calcination at 800° C. for 3 h, the tungstated Al³⁺ doped zirconia obtained was impregnated with 0.5 wt % Pd using an aqueous [Pd(NH₃)₄](NO₃)₂ solution. The catalyst was calcined at 450° C. for 3 h before it was placed into the reactor.

Example 9

Catalytic Isomerization Test:

Prior to the catalytic experiment, the catalyst was reduced with H₂ (200 Nml/min) at 200° C. and atmospheric pressure. Heptane isomerization with the catalyst prepared according to Example 8 was performed in a fixed-bed reactor at a total pressure of 6 atm and with a LSHV of 2.14 h⁻¹. The feed was consisting of a hydrogen/hydrocarbon mixture with a molar ratio of 4.93:1. Catalytic results are shown in Table 1. TABLE 1 Selectivity to Catalyst Amount of Carrier 2,2-DMP, prepared Al₂O₃ in calcination C₇ Cracking 2,4-DMP and in sample, temperature, Temperature, Conversion, factor, 2,2,3-TMB, Example: (wt %) (° C.) (° C.) (%) (%) (wt %)  2 22 650 200.00 59.8 5.2 6.55 4a 14 600 200.50 31.0 0 1.50 4b 14 650 201.00 47.9 3.36 3.95 4b 14 650 211.60 67.5 6.55 8.98 4c 14 700 200.00 87.4 27.7 19.1 4d 14 800 200.00 90.4 61.5 20.2 4d 14 800 162.20 40.0 9.50 4.12  6 0 825 230.00 7.9 23.8 0.09  8 3.2 800 165.0 27.2 9.8 2.38  8 3.2 800 178.0 51.9 13.2 6.30  8 3.2 800 205.0 88.3 38.2 18.96

Example 10

Preparation of an Isomerization Catalyst from Spray Dried Product pre-calcined to 700° C.:

1.00 kg of the spray dried product obtained in Example 1 was calcined at 700° C. for 6 h. 350 g of the calcined powder was mixed with 250 g alumina gel (pseudoboehmite 30%) and 50 g pseudoboehmite powder for 10 min and extruded as 1/16″ cylinders. The extrudates were dried at 110° C. over night and calcined at 650° C. for 3 h. 0.5 wt % Pd was introduced by incipient wetness impregnation of an aqueous [Pd(NH₃)₄](NO₃)₂ solution. The thus obtained catalyst was calcined at 400° C. for 6 h in a flow of air (4 l/min·kg catalyst) before it was placed into the reactor.

Example 11

Catalytic Isomerization:

Prior to the catalytic isomerization experiment, the catalyst was reduced with H₂ (200 Nml/min) at 200° C. and atmospheric pressure. Heptane isomerization with the catalyst prepared in Example 10 was performed in a fixed-bed reactor at a total pressure of 6 atm and with a LSHV of 2.14 h⁻¹. The feed consisted of a hydrogen/heptane mixture with a molar ratio of 4.79:1. Catalytic results are shown in Table 2. TABLE 2 Selectivity to Catalyst Amount of Carrier 2,2-DMP, prepared Al₂O₃ in calcination C₇ Cracking 2,4-DMP and in sample, temperature, Temperature, Conversion, factor, 2,2,3-TMB, Example: (wt %) (° C.) (° C.) (%) (%) (wt %) 10 24 650 209.60 59.1 8.82 6.29 

1. Process for preparation of a paraffin isomerization catalyst comprising a mixture of a Group IVB metal oxide, a Group VIB metal oxide, a Group IIIA metal oxide and a Group VIII metal, said process comprising the steps of (a) contacting a hydroxide of the Group IVB metal with an aqueous solution of an oxyanion of the Group VIB metal to provide a mixture, (b) drying the mixture to provide a dry powder, (c) kneading the dry powder with a Group IIIA hydroxide gel to form a paste, (d) shaping the paste to form a shaped material, (e) calcining the shaped material, (f) impregnating the calcined material with an aqueous solution of a Group VIII metal salt to provide the catalyst, (g) calcining the catalyst.
 2. Process according to claim 1, wherein the dry powder is calcined before kneading.
 3. Process according to claim 1, wherein the Group IVB metal is zirconium, the Group VIB metal is tungsten and the Group III A metal is aluminum.
 4. Process according to claim 3, wherein the catalyst comprises tungsten oxide, aluminium oxide, zirconium oxide and the Group VIII metal.
 5. Process according to claim 4, wherein the catalyst in its dry form comprises 10-50 wt % tungsten oxide, 10-40 wt % aluminium oxide and a remainder of zirconium oxide and the Group VIII metal.
 6. Process according to claim 1, wherein the Group VIII metal is palladium and/or platinum in an amount of 0.01-5 wt %.
 7. Process according to claim 1, wherein the mixture is dried by spray drying and the paste is shaped by extrusion to provide an extrudate.
 8. Method for the production of high-octane gasoline from a hydrocarbon feedstock comprising C₅₊ hydrocarbon cuts, wherein the feedstock is contacted with a paraffin isomerization catalyst prepared according to claim
 1. 9. Method according to claim 8 wherein the hydrocarbon feedstock comprises C₇₊ hydrocarbon cuts or cuts containing this fraction. 